Intake Air Temperature Sensor

In the old air filter box, there is an inlet air temperature sensor. When I replaced the air filter box with a freer flowing air filter, I wasn't sure what to do with the inlet air temperature sensor. I orginally mounted it to close to the air filter using tie wraps. It tracks the MAF air inlet temperature for the computer. I understand that it's job is to keep the intake air temperature close to 70 degF. The MCU does this by opening and closing a valve that lets in hot air from the exhaust heat riser or fresh air from the cold air intake.

Air inlet temperature sensor mounted close to air filter

The problem that I ran into was that as soon as the air temperature got around freezing (32 degF), the gas mileage dropped drastically down to 15 mpg. The engine's response seemed to be all over the place - one day very responsive and the next a dog. I tried tying it to a heater hose and that was a BIG mistake, the engine's response nose dived and so did gas mileage.

I initially replaced the inlet air temperature sensor with a resistor ($0.10) and drove it that way for almost 3 years. Those modifications are documented further down this page. During that time, I did many modifications to the engine and the truck to improve performance and gas mileage, they are listed throughout this site. I live in Canada and the temperature varies all year round and in the Fall, it can very greatly from early morning (just above freezing) to mid afternoon heat (80 degF). I thought that now would be a good idea to revisit the inlet temperature sensor and mount it in the intake tube.

Air inlet temperature sensor mounted on the intake tube

It was actually pretty easy to mount. There is a plastic nut that unscrews and then you are left with a 11/16" dia thread. I used a 5/8" diameter knock-out punch to create the hole and the sensor screwed in like a self tapping screw. I made a rubber gasket and coated the intake side with Permatex black instant gasket. Hooked everything up and was done.

I must say the engine runs way smoother at all temperatures now compared to when I had it with just a resistor. I think that it is a combination of modifications that work now. Before the resistor made sense as an inexpensive method of improving performance.

Replacing Inlet Temperature Sensor with a Resistor

I disconnected the sensor and measured the resistance across the leads and found that at 72 degF, it read about 30 kohms. So I swapped it out for a 30 kohm resistor. Drove it for a couple of days and the truck felt sluggish. I talked to my nephew and he suggested that a resistor simulating about 65 degF might work better. It just happened that my garage was about 60 degF that day so I remeasured the resistance and it was close to 43 kohm. Replaced the sensor with the new resistor and the engine runs much better and smoother.

Air inlet temperature sensor - tip is the actual sensor

The sensor is very sensitive and responds surprisingly quickly to temperature changes. Just touching the tip with your fingers will bring an instant reading change.

Air inlet temperature sensor beinge tested with digital ohmeter

10 cent 43 kohm resistor soldered to the sensor leads (sorry for the picture quality). The air filter doesn't look so shiny and new after a year on the road!

The resistor protected with electrical tape then taped to the MAF wiring.

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Copyright February 2011 Eugene Blanchard